1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) having a storage capacitor and a method of manufacturing thereof. More specifically, the present invention relates to a structure and method for improving the image quality of an LCD by reducing the fluctuations in the capacitance of a storage capacitor of an LCD.
2. Discussion of the Related the Art
A thin film transistor (TFT) LCD device includes TFTs for use as switching devices, capacitors being defined by liquid crystals disposed between upper and lower plate electrodes, subsidiary capacitors, gate lines, and data lines.
For driving a TFT-LCD, a signal voltage is applied to a gate electrode, the TFT then turns on so that a data signal including image data is transmitted to the liquid crystals through the TFT. The liquid crystals that are within the electrode plates of the capacitors are charged. Ideally, the total electric charge that is stored in the liquid crystals remains constant until the next signal is applied.
However, the liquid crystal voltage varies due to the existence of various sources of capacitance and the voltage varies by an amount xcex94V, which is expressed by the following approximate formula: xcex94V=Cgd*Vg/(Cgd+CLC+Csto), where xcex94V is the maximum amount of variation of liquid crystal voltage, Cgd is parasitic capacitance due to the overlap between the gate and drain electrodes, CLC is liquid crystal voltage, Csto is the capacitance of a storage capacitor, and Vg is the voltage of the gate electrode. The existence of xcex94V causes distortion in the liquid crystal voltage and is the primary reason for flicker in images produced on the LCD device. To decrease xcex94V, it is preferable to increase the capacitance of the storage capacitor Csto.
FIG. 1 shows an arrangement of an LCD device having a gate storage capacitor. Referring to FIG. 1, a pixel is defined on a substrate (not shown) by the intersection of the gate lines 10L and 11L and the data line 12L. A gate electrode 11G is connected to the gate line 11L, a source electrode 12S is connected to the data line 12L, and a drain electrode 12D is arranged so as to oppose the source electrode 12S. An active layer 15 is overlapped with the above-mentioned three electrodes and constitutes a TFT for use as a switching device. A pixel electrode 17 is connected to the drain electrode 12D and covers the pixel area.
A portion of the (nxe2x88x921)th gate line 10L and a portion of the nth pixel electrode 17 are overlapped and define a storage capacitor. In general, a gate insulating layer and a passivation layer defines a dielectric layer of the storage capacitor and the gate line and the pixel electrode define the electrodes of the storage capacitor.
But, when a subsidiary electrode is connected to the pixel electrode and disposed on the gate insulating layer as shown in FIG. 2, the gate insulating layer functions as a dielectric layer since the subsidiary electrode and gate line also define electrodes of a storage capacitor. In the above-described case, the capacitance can be increased since it is possible to reduce the thickness of the dielectric layer.
FIG. 2 shows a layout view of a storage capacitor in an LCD device according to a related art. FIG. 3 shows a cross-sectional view of a storage capacitor in an LCD device according to a related art. Referring to FIGS. 2 and 3, a gate line 21L including a gate electrode 21G is provided on a substrate 200, and a gate insulating layer 22 is provided on an exposed surface of the substrate including the gate line 21L. A subsidiary electrode 23, which is preferably made of a metal that is used to form the source/drain electrode, is defined on the gate insulating layer 22. A passivation layer 24 covers the subsidiary electrode 23, and a contact hole (not shown in FIG. 2) is defined on the passivation layer 24 and exposes a portion of the subsidiary electrode 23. A pixel electrode 25 is connected to the subsidiary electrode 23 through the contact hole that is defined on the passivation layer 24.
In the above-described structure, storage capacitance of a storage capacitor is provided by the subsidiary electrode 23, a portion of the gate line 21L that is overlapped with the subsidiary electrode 23, a portion of the pixel electrode 25 which is not overlapped with the subsidiary electrode 23, and a portion of the gate line 21L that is overlapped with the pixel electrode which portion is not overlapped with the subsidiary electrode.
For the sake of explanation, the term xe2x80x9cstorage capacitorxe2x80x9d in this specification is defined as the subsidiary electrode 23 and a portion of the gate line 21L which is overlapped with the subsidiary electrode in the present specification.
The capacitance of a storage capacitor is expressed as xe2x80x98Cxe2x88x9dA/dxe2x80x99, where A is the overlapped area between the subsidiary electrode and the gate line, which are two electrodes of the storage capacitor, and d is the thickness of the gate insulating layer, which defines the dielectric layer between the electrodes. Unfortunately, it is difficult to increase the area of the subsidiary electrode due to the structural limitations of the LCD of the related art. Thus, in the related art, it is very difficult to increase the capacitance of the storage capacitor. Accordingly, the conventional LCD device has very poor image quality due to flickering and other image defects.
To overcome the problems described above, preferred embodiments of the present invention provide a storage capacitor in an LCD device that improves the image quality by increasing the capacitance of the storage capacitor to reduce the fluctuations in the liquid crystal voltage.
Further, preferred embodiments of the present invention provide a storage capacitor in an LCD device that improves the image quality by increasing the capacitance of the storage capacitor by increasing the total exposed surface of the gate line. The total exposed surface of the gate line can be increased by increasing the total number of lateral surfaces in the gate line. The total number of lateral surfaces of the gate line can be increased by one or more open portions in the gate line.
Further, preferred embodiments of the present invention provide a storage capacitor in an LCD device that improves the image quality by increasing the capacitance of the storage capacitor by increasing the area of overlap between the gate line and subsidiary electrode.
A preferred embodiment of the present invention includes a substrate, a gate line on the substrate and including at least one open portion on a surface of the gate line, the open portion defining additional lateral surfaces of the gate line, a gate insulating layer covering the gate line, and a storage capacitor electrode on the gate insulating layer wherein the storage capacitor electrode is overlapped with the gate line.
In another preferred embodiment of the present invention, a method of manufacturing an LCD device includes the steps of providing a substrate, forming a gate line having at least one of open portion on a surface of the gate line, forming a gate insulating layer covering the gate line, and forming a storage capacitor electrode on the gate insulating layer to be overlapped with the gate line.
Therefore, preferred embodiments of the present invention improve the image quality of an LCD display device by reducing flickering and other image defects that are caused by fluctuations in the liquid crystal voltage by increasing the capacitance of a storage capacitor of the LCD display device.
Other elements, features, details and advantages of the present invention will become apparent from the detailed description of preferred embodiments of the present invention in conjunction with the attached drawings.